The present invention relates to method and apparatus for measuring cloud altitude, and particularly to such means in which the sensitivity of a receiver for detecting echo signals from the clouds is controlled to adjust the receiver for the detection of only clouds, not haze, etc.
A device which is particularly suitable for measuring the cloud altitude is known from U.S. Pat. No. 3,741,655. The known device comprises measuring equipment of the optical radar type, which means that a transmitter emits short light pulses directed towards an object. In this case the object is a cloud, and when such light pulses hit the cloud reflexes occur, and part of the reflected light is intercepted by a receiver located adjacent to the transmitter. The time required for the light to travel the distance, transmitter-cloud-receiver, is measured and the altitude of the cloud is determined with the known velocity of light as the starting point. The known device according to U.S. Pat. No. 3,741,655 further comprises two integrating devices in the receiver unit, which are alternately caused to receive signals intercepted by the receiver. One of the integrating devices is designed to receive echo signals expected to be emitted from clouds, and the other integrating device is designed to receive only noise signals. After a number of light pulses have been emitted and echo signals have been received, the contents of the integrating devices are compared, and the result of the comparison is placed in proportion to a predetermined signal level and, if this level is exceeded the existence of clouds is indicated.
In this kind of cloud altitude measuring a well-adjusted sensitivity control of the receiver is necessary in order that precipitation and haze shall not be detected as a cloud and give rise to incorrect information as to the height of the cloud base. It is normal in this connection to let the sensitivity of the receiver increase, in accordance with a given function, with the distance from the receiver from which an echo signal is expected.
When the transmitter and the receiver, as in the above example, are placed adjacent to each other, this causes the area nearest the device to become less sensitive to the device, and echo signals with very low level are received from that area.
In the accompanying drawing, FIG. 1 illustrates the above problem. The letter S designates the transmitter, S1 and S2 the limits of the transmitter beam, M the receiver and M1 and M2 the limits of the field of view of the receiver. The boundaries S1, S2 and M1 and M2 show only an ideal condition. Within the transmitter beam and the field of view of the receiver there is in practice a distribution of the intensity, which diminishes to the sides of the beam. From the area A, the receiver only receives signals of a very low signal level, which is due to the fact that such low level signals lie outside the real field of view of the receiver. It is possible, therefore, that echo signals from clouds located at a very low height will not be indicated. FIG. 2 shows the section 1--1 of FIG. 1, where the area D shows the part of the transmitter beam which, in section 1--1, lies within the field of view of the receiver.